Presently, the optical disc (in other words, optical information disc) such as a DVD (Digital Versatile Disk) or CD (Compact Disc) is widely spread in the market.
Ordinary, DVD is in the range of protective substrate thickness is 0.6 mm, and the wavelength of the laser light is 635 nm-675 nm, and CD is in the range of protective substrate thickness is 1.2 mm, and the wavelength of the laser light is 765 nm-805 nm, however, there is a production error in the protective substrate thickness of these optical discs, and as an optical pickup apparatus to record or reproduce them, it is considered that it is more desirable that, even when there is more or less fluctuation in the protective substrate thickness of the optical disc, the apparatus can correspond enough to it.
Other than such a more or less fluctuation in the protective substrate thickness, in the case where it is intended that the production cost of the optical disc is more reduced, it is also considered in the future that the material consumption of the protective substrate is reduced by the means that the protective substrate thickness of, particularly, CD whose protective substrate is 1.2 mm and thick, is reduced as possible. Further, for example, there is a possibility that the optical disc in which the DVD and CD are integrated is assumed in the future, and it can also be assumed that, in that case, the protective substrate thickness of the CD is reduced (for example, the protective substrate thickness is about 0.9 mm) for suppressing the thickness of the optical disc. (Hereinafter, although the substantial standard is the same as an ordinal CD, the CD whose protective substrate thickness is about 0.9 mm and very thin, and which is out of the initial standard, is called herein as “new format optical disc”).
In the future, in the case where it is assumed that this new format optical disc is spread in the market, the development of the compatible-use optical pickup apparatus by which the reproducing and/or recording can be conducted not only for the existing DVD or CD but also for the new format optical disc, is important, and particularly, one lens system optical pickup apparatus by which the compatibility is conducted by one objective lens is the most ideal mode.
In the optical pickup apparatus by which both of DVD and CD can be reproduced and/or recorded, at the time of reproducing/recording of CD, there is no necessity that the ray of light passing an area whose numerical aperture NA is larger than about 0.45 in the optical surfaces of the objective lens is used.
Accordingly, by using the diffractive structure formed on the optical surface of the objective lens, on the one hand that the laser light for CD passing the area whose numerical aperture NA is larger than about 0.45 is made so-called a flare light on the information recording surface of CD, a technology that the laser light for DVD is made converged with about no aberration on the information recording surface of DVD, is well known (for example, refer to Patent Document 1).
[Patent Document 1] Tokkai No. 2001-195769
Generally, when a light flux passes a diffractive structure, diffracted light fluxes with a plurality of diffraction orders are generated, and in these diffracted light fluxes, the recording and/or reproducing of the optical disc is conducted by using the diffracted light flux whose diffraction efficiency is the highest. However, there is a problem that the remaining diffracted light fluxes whose diffraction efficiencies are low and which are not used, (hereinafter, called as “unnecessary diffracted light fluxes”) badly affect the operation of the optical pickup apparatus. Specifically, when a position at which the unnecessary diffracted light flux passes the objective lens and crosses the optical axis, and a position at which the usable diffracted light flux passes the objective lens and crosses the optical axis, are overlapped, the spot diameter of the converged spot is enlarged on the information recording surface of the optical disc, and there is a problem that apparent numerical aperture NA is lowered. Particularly, when the paraxial light-converging position of the unnecessary diffracted light flux is overlapped with the paraxial light-converging position of the usable diffracted light flux, the phases of both diffracted light fluxes coincide with each other at the converged spot, and the problem of the spot diameter enlargement is more conspicuous.
Further, when the position at which the unnecessary diffracted light flux passes the objective lens and crosses the optical axis is in close vicinity to the position at which the useable diffracted light flux passes the objective lens and crosses the optical axis, there is a case where the unnecessary diffracted light flux reflected on the information recording surface of the optical disc enters into a sensor for reflection light detection, noises are mixed in the RF signal, and the reading error of the recording and reproducing signal is generated. The RF signal is a signal used for the focus detection by the astigmatism method, and the returning characteristic from the sensor for the dislocation (ΔfB) from the best focus position is shown. The focus detection is conducted by using the linearity of this RF signal.
Because an interval in the optical axis direction of the paraxial light-converging position of the diffracted light flux depends on the power of the diffraction, the chromatic aberration of the using laser light flux is influenced.
Further, the technology disclosed in Patent Document 1 is one for attaining the compatibility between two kinds of optical discs of DVD and CD, and not only DVD and CD, as the technology for attaining the compatibility among three kinds of optic al discs including the new format optical disc as described above, it is difficult that the technology disclosed in the Patent Document 1 is applied as it is.